It was noted that the majority of samples of MgB2 exhibited resistivities that were much higher than the low values that were seen in a relatively small number of single crystals, bulk samples and films. In many cases, the resistivity was increased - over the single-crystal values - by some orders of magnitude. Even at these high values, there was often still a metallic temperature dependence of the resistivity, and TC was often surprisingly near to the bulk value of 39K. The resistivity increase was ascribed to a reduction in the effective current-carrying cross-sectional area of the sample. However, if this loss of cross-sectional area were the dominant factor contributing to the increase in resistivity of samples, then the critical current density had to be decreased by the same loss in the effective area. Comparisons of MgB2 resistivity with the properties of disordered cluster-compound superconductors, HTS materials and granular Al (heavily contaminated with O), revealed similarities between MgB2 behavior and the results of HTS and Al studies. Various effects were considered that might contribute to the reduction in effective sample area in MgB2 samples, to the increase in resistivity and to a reduction in JC. It was speculated that a Josephson junction model of the grain boundaries in MgB2 might apply to samples with extremely high resistivities. Alternatively, it was suggested that the 2-band nature of MgB2 could result in an unusual behavior of its resistivity and TC as the material changed from clean to dirty. It was concluded that measurements of both the resistivity and the transport critical current density for a wide variety of bulk and film samples would be very useful, as would a study of resistivity and JC changes as a function of irradiation damage in MgB2 bulk and films of low initial resistivity.

The Widely Variable Resistivity of MgB2 Samples. J.M.Rowell: Superconductor Science and Technology, 2003, 16, R17-27